When feeding AC power through long cables in transmission systems feeding an electrical load (machines/or other loads), the voltage at consumer end will be heavily influenced by electrical load drawn by the consumer. An example of such a long cable is a cable some kilometers in length connecting a power supply on land to a transformer or to a motor of a pump installed out to sea. In the field of Oil and Gas exploration and production, such loads or machines may be for example be mounted on the seabed tens of kilometers away from land or a topside platform. Electrical equipment such as a subsea multiphase pump or pressure booster pump or a subsea compressor used in Oil and Gas production or transfer installations may be operated underwater, eg on the seabed, at depths 1000 meters, or more.
When a power cable becomes very long and/or frequency becomes high, the voltage becomes even more dependent on the electrical load drawn by the consumer or consuming unit, and at a point it is not possible to keep the voltage within acceptable limits without doing active actions, examples of which may be:
1. Active control of the voltage at the motor terminals by using measured motor voltage feedback with regulation of voltage at sending end of the cable;
2. Control of reactive power by means of adding reactors at the sending end, or receiving end, Static Var compensation (fast-acting reactive power compensation), or similar equipment.
One solution would be to measure the voltage in the transmission end, ie at the load end, and use this as feedback for closed-loop control of the voltage. However, this may become a challenging and more costly solution especially for subsea systems, since this will the lead to additional equipment being installed subsea. Another technical challenge is additional penetration of subsea equipment and measurement transfer link the converter control system. Water penetration at sea and water pressure at subsea depths are added technical issues. This type of solution may thus create technical challenges and questions with relation to reliability of the system in case of component failure.
JP5122806 describes a method for controlling a linear synchronous motor for a railroad vehicle. The method includes calculating a correction for voltage drop due to vehicle position along a length of feeder line.
US2009/0256519 discloses a method and program for cable loss compensation in an electrical submersible pump system. This document discloses a submersible electrically driven pump in a wellbore. The pump is supplied by a voltage controller, which communicates with a current sensor, and which adjusts the power source output using sensor measurements to calculate a voltage drop as the product of current and cable impedance.